Both epidemiological studies in humans and laboratory studies with animals have provided evidence that high fat diets enhance mammary tumor growth. The data indicate that this effect of lipids on breast cancer is due to an enhancement of cell growth during the "promotional" phase of tumor development and, that a proper hormonal environment is necessary for its maximal expression. The biochemical processes responsible for this lipid-mediated enhancement of tumor growth are poorly understood, but one promising possibility is that qualitative and quantitative differences in dietary lipids may induce alterations in the physical-chemical properties of specific cell membranes which, in turn, allow a significant amplification of hormonally mediated stimuli. It is the objective of this study to examine this hypothesis by attempting to correlate the gas chromatographic patterns of mammary tumor membrane lipids of animals on high and low fat diets with their tumor growth and hormone receptor activity. Furthermore, this study will also attempt to establish whether these lipid effects on tumor growth can be modified by specific hormone excesses or deficiencies. Ultimately, it is hoped that such information will extend our knowledge of the tumor promotion process far enough to enable others to develop therapeutic programs capable of inhibiting or, at least, delaying breast cancer growth.